Generally, polyurethanes are multi-block polymers, usually consisting of soft segments derived from polyols, and hard segments arising from isocyanates and extenders. Because the polyols generally constitute the majority of the formulation weight, the final properties of polyurethane products are largely determined by the polyols. Therefore, much attention has been paid to the polyol manufacture. A great polyol variety can be used to build the polyurethane polymeric architecture, and the most widely employed polyols are polyether and polyester polyols. However, they are plagued by a number of demerits and have limited applications in some fields. For example, although polyether based polyurethanes exhibit good stability against hydrolysis, they suffer from poor strength properties and heat resistance. In contrast with these, polyester based polyurethanes show good mechanical performance and heat stability, however, the susceptibility to hydrolysis is a primary problem requiring solution.
To integrate respective merits and remedy the respective disadvantages originating from the conventional polyether and polyester polyols, both ether and ester linkages containing polyetherester polyols enjoy growing interest recently. They are expected to have a more attractive performance than the separate polyether or polyester polyols. Furthermore, the polyetherester polyols could be used alone, or in combination with the conventional polyols for polyurethane production, therefore extending the polyol source.
Simple blending of a polyether and a polyester is not an option for solving the mentioned problems, due to their incompatibility in nature. To date, various synthetic routes for the preparation of different polyetherester polyols have been proposed. Some representative methodologies are summarized as follows: (1) catalytic addition of alkylene oxides onto polyester polyols using double metal cyanide complex (DMC) catalyst, for example see U.S. Pat. No. 6,753,402; (2) Polyether reacts with dicarboxylic acid or anhydride with the aid of different catalysts to implement the insertion of anhydride into carbon-oxygen bonds of polyether, as disclosed in U.S. Pat. Nos. 5,319,006, 5,436,313, and 5,696,225; (3) Anhydride first reacts with diol to form an intermediate polyester polyol, followed by the reaction with an alkoxylating agent, for example see U.S. Pat. No. 6,569,352; (4) Copolymerization of CO2, alkylene oxide, initiator, for example see U.S. Pat. Application 20080021154; (5) Copolymerization of anhydride, alkylene oxide and alcohol initiator, as described in U.S. Patent Application No. 20070265367 and by D X Wang et al in “Synthesis, Characterization, and Properties of Novel Polyetherester Polyols and Developed Polyurethanes” J Appl Polym Sci vol 103, 417-424 (2007). (6) Reaction of hydroxyl group-containing monocarboxylic acid esters and/or polycarboxylic acid esters with alkylene oxide followed by transesterfication as described in U.S. Patent Application No. 20060211830.
In addition, polyetheresters can be produced by the copolymerization of alkylene oxide and cyclic esters such as lactone in the presence of a suitable initiator, as described in U.S. Pat. No. 5,032,671 or U.S. Patent Application 20070088146 or International Application No. 2007020879 and references cited therein. In the preparation of such polyetheresters, catalyst choice is crucial.
DMC complex is known as a catalyst with an extraordinarily high activity for alkylene oxide polymerization. The polyether polyol gained thereby is characterized by low unsaturation and narrow molecular weight distribution (MWD) in comparison with common polyether synthesized using a traditional KOH catalyst. Besides, it can be used to produce other polymers encompassing polyester polyols and polyetherester polyols. Recent improvements in preparative methodology have made DMC catalyst much more attractive for commercial manufacture.
Though DMC has proved very efficacious for the copolymerization, drawbacks still exist. For example, the catalyst is hindered by the observation that the products obtained often are turbid or layered. Such products are incompetent because the inhomogeneity not only affects the appearance, but also damages the performance of final polyurethane materials (see Example 5 below). It is demonstrated by our experiments that the undesirable phenomena are closely associated with the catalyst employed.
Thus, the problem to be solved is to provide a modified catalyst system for use in a process for the production of polyetherester polyols. Preferably, the catalyst would be successful in the synthesis of uniform polyetherester polyols. Uniform here means that the monomers are evenly distributed over the polymer chain, the chain being free of defects. Defects could arise from e.g. non-complete conversion or side reactions like e.g. decarboxylation leading to polyols that have a real functionality less than the theoretical value. The uniform telechelic polyether-ester polyols that are free from chain defects would preferably also show improved mechanical properties and swelling characteristics when applied in making a polymer.